Only about one-half witnessed out-of-hospital cardiac arrests are seen by bystanders in CPR. Therefore, the number of centralized centers for dispatch CPR services is growing. Ad hoc efforts to provide first aid instructions over the telephone for the relatively complicated procedures involved in CPR was only recently attempted. A study conducted in Seattle in 1975-76 was unable to demonstrate that the quality of the bystander CPR was related to survival rate, suggesting that a simpler message involving chest compressions only (CC) might be adequate. The past 3 years we developed and evaluated an interrogation protocol for EMS and two alternate methods of CPR instructions: One involving airway, breathing and compression (ABC) and the other chest compression only (CC). Over 1000 dispatches were monitored. The study showed both instructions are safe. However, ABC is more difficult to communicate and takes more time. The proposed research is a randomized clinical trial (RCT) of whether CC is as efficacious as ABC. We have developed the concept of computer aided interrogation and diagnosis (CAID). Subsequent to diagnosis, the computer can provide guidance for instructions about medical assistance. The RCT is enabled by random computer allocation of one of the instruction sets. We also propose to enhance the capabilities of the CAID, e.g., to CPR for children and infants, seizures, respiratory arrest. CAID will be a useful tool for training and for dispatching. Practical implications of the RCT are: If CC is inferior, optimizing the ABC protocol and better methods of instruction in its use should be persued; if CC is as effective as (or better than) ABC, the former would be the choice for pre-arrival instruction and possibly an alternative for citizen CPR training.